Figure 11-3. Antigen presenting cell with signals, T cell and receptors. Redrawn from Medzhitov and Janeway with permission; see that reference for details. Original figure copyright 2000, Massachusetts Medical Society, all rights reserved.

Antigen-MHC complex

Activation

Antigen-presenting cell

Figure 11-3. Antigen presenting cell with signals, T cell and receptors. Redrawn from Medzhitov and Janeway with permission; see that reference for details. Original figure copyright 2000, Massachusetts Medical Society, all rights reserved.

There are approximately 30,000 TCRs on the surface of every T cell. There are two types of these receptors, both formed of heterodimers composed of two polypeptide chains joined via a disulfide bond (Janeway 2001).The chain of the predominant type of receptor is made of a- and b-subunits, whereas the second, and less common, type of TCR , is a dimer of g- and S-chains. These receptors recognize antigen differently, and in fact, the role of the TCR is not yet fully understood. These heterodimers are physically associated on the T cell surface with five polypeptides called the CD3 TCR complex, which is required for the expression of the TCR at the cell surface, although it is not yet clear how. Ninety to ninety-five percent of T cells are ab T cells and the rest are gS T cells (Roitt, Brostoff et al. 1998).

Leukocytes express many different molecules on their surfaces, which are used to classify cell types by a cluster designation (CD) type. ab T cells are classified as CD4+ ("helper") or CD8+ ("killer") cells. Helper T cells activate other cells to destroy pathogen, whereas CD8+ T cells kill directly. Although gS T cells comprise a small fraction of circulating T cells they are frequent in mucosal epithelia. It is thought that these T cells may be important in protecting mucosal surfaces from some kinds of viral and bacterial infections.

The Major Histocompatibility Complex

The family of genes involved in tissue rejection comprises the MHC. This was first described in the mouse; the human analog is called the human leukocyte antigen (HLA) system (the organization of both systems is shown in Figure 11-4). Histo-compatibility antigens are encoded by histocompatibility genes. The HLA complex is on human chromosome 6, and includes more than 200 genes, 40 of which encode leukocyte antigens. These are among the most polymorphic of all mammalian genes